JPS62250158A - Steel for hot forging molds - Google Patents

Abstract

PURPOSE:To obtain a steel for hot forging die improving the service life of die at relatively low temp., by limiting respective quantities of C, Si, Mn, P, S, Cr, Mo, V, and Al and also by controlling Ni content in correlation with the surface temp. of a die in the course of hot forging. CONSTITUTION:The steel for hot forging die is composed of an alloy steel which has a composition consisting of, by weight, 0.50-0.70% C, <=0.50% Si, 0.20-0.90% Mn, <=0.035% P, <=0.015% S, 0.50-2.0% Cr, 0.20-0.60% Mo, 0.04-0.30% V, <=0.10% acid-soluble Al, and the balance Fe with inevitable impurities and further containing Ni so that -0.025T+19<=Ni<=-0.025T+21 (where T means the surface temp. of a die in the course of hot forging, deg.C) is satisfied. This steel has superior wear resistance and toughness in use under the condition of <about 750 deg.C die-surface temp. and it is excellent in cracking resistance as well.

Description

[Detailed description of the invention] Birth! Field of use of Jk The present invention relates to die-molded steel used for hot forging.

Conventional technology Conventionally, materials for hot forging molds have focused on increasing high-temperature strength at the average operating temperature of the mold and improving hot wear resistance, and materials such as Mo have been used. Component designs have been made mainly to increase strength by adding precipitation-strengthening alloying elements.

Purpose of the Invention However, when alloy steel, which has increased high-temperature strength and improved hot wear resistance by the above-mentioned conventional Mof)'th addition, is used as a material for a mold, the toughness decreases as the strength increases. As a result, cracking resistance is low, and cracks that penetrate through to the inside often occur, especially in large molds.

The present invention solves the above-mentioned conventional problems, and the mold surface temperature is 75%.
The object of the present invention is to provide a steel for hot forging molds that has improved mold life when used at temperatures below 0°C.

Structure of the Invention In order to solve the above-mentioned conventional problems, the inventors of the present invention have found that, after conducting various studies, it is necessary to add Ni, which does not belong to precipitation-strengthening alloying elements such as Mo, but stabilizes the ausbunite phase. It has been found that the strength at high temperatures is increased by oxidation of the surface layer of the mold, and Ni is concentrated in the surface layer, particularly strengthening the surface layer, and the wear resistance in hot conditions is improved. The present invention was made based on the above findings, and includes C: α50 to α70%, Si: α
50% or less, in: α20 to α90%, P: 0.03
5% or less, S: α015, % or less, C'r: α50-
40%, Mo: α20-0.00%, V: 0.04-α
30%, acid solubility, l (hereinafter referred to as Fsoff1.Aj2): Contains 0.10% or less, further Ni, and T is the surface temperature If (%) of the mold during hot forging, -α025
T+19≦Ni≦~α025T+21 for 1fl
J, Yosha (1), and the remainder is Fe12 and unavoidable impurity alloy steel.

According to this invention, its characteristics are exhibited when the mold surface temperature is lower than 750°C.

The above-mentioned mold surface temperature is the temperature of the extremely thin surface layer of the mold during forging, and is the temperature determined correspondingly from changes in the microstructure of the surface layer.

Below, we will explain the composition of the alloy steel that is the material for the mold and the reason why the temperature at which the mold is used was limited to the values listed in E.

First, component C is effective in increasing strength, but it is 0% by weight.
．． If it is less than 50% (hereinafter simply referred to as %), sufficient strength cannot be obtained, while if it exceeds 0.70%, the toughness and the cracking resistance of the mold will decrease, so the content should be reduced to 0.50%.
It was limited to ~α70%.

The Si component is necessary for deoxidation and also works to ensure strength, but if it exceeds α50%, it will deteriorate the toughness.
Its inclusion because it provokes enemies! Positive is defined as α50% or less.

The Mn component has the effect of improving hardenability and improving strength and toughness, but if α is less than 20%, martensite will be formed in mm and the desired strength cannot be obtained, and if α exceeds 90%, toughness will decrease. Therefore, the content is α20 ~
It was limited to α90%.

Both P and S components are impurities that are unavoidably contained, and reducing their content is extremely effective for strengthening, so the P component is 0.035% or less, and the S component is α015% or less. It is better to keep the content of both of them as low as possible.

The Cr component has the effect of improving strength and toughness, but if α is less than 50%, the desired strength cannot be obtained;
If the content exceeds α50, the toughness will deteriorate.
% or less.

The Mo component has the effect of improving strength and toughness like the C component, but if it is less than 020%, the desired strength cannot be obtained, and if it exceeds α60%, the toughness deteriorates, so the content should be reduced from α20 to It was limited to α60%.

■Although the ingredients also have the effect of improving strength and toughness,
If α is less than 04%, the improvement effect is not sufficient, and if it exceeds α030%, the toughness will deteriorate, so the content should be reduced to 0.04%.
It was limited to ~α30%.

AJ2 component is added for deoxidation, but the content is α1
If the content exceeds 0%, the toughness decreases, so the content is
oi, AIlα was set at 10% or less.

As mentioned above, the Ni component has the effect of increasing the strength at high temperatures, and the oxidation of the surface layer of the mold causes Ni to become concentrated in the surface layer, causing the surface layer to form an austenite phase with higher strength than the ferrite phase at high temperatures. The surface layer of the die is particularly strengthened by changing the Ni content to a specific value determined depending on the die surface temperature during forging (hereinafter referred to as If the critical Ni content is exceeded (called the critical Ni content), the ductility of the surface layer of the mold will decrease significantly and separation mainly due to brittle cracks will occur.
1... (Otori) However, T was limited to a range that accommodates the mold surface temperature (°C) during forging.

Figure 1 shows the relationship between Ni content and elongation of gold on the surface! ! This is a diagram showing the results of investigation assuming different degrees of strength, where the horizontal axis shows the Ni content and the vertical axis shows the elongation. In the figure, when the actual heddle tensile test temperature is 700℃ and the mold surface temperature is relatively low at 700℃, the broken line indicates the tensile test temperature is 780℃.
This is a case where the mold surface temperature corresponds to a relatively high 780°C. In the figure, the amount of Ntffi exceeds the critical Ni content. However, the critical Ni content is a5% when the mold surface temperature is 700°C, and the elongation decreases rapidly.
%, it is 1.5% and decreases as the mold surface level increases.

Now, if we set this relationship as a linear formula% formula% y=1.5 and find a and b, a=-00
25, b=21, and the equation (2) becomes an equation showing the upper limit of the equation (1) showing the range of the Ni content. Also, from the example described later, the lower limit of Ni at a mold surface temperature of 700°C is 1.5%, so in the above equation (2), χ = 7
00, y=t, 5 and assuming that the temperature gradient & does not change, set a=-0,025 and find b, then b=19.
Equation (2) represents the limit of Equation (1), which indicates the range of the Ni-containing range.

Next, regarding the applicable range of the mold usage temperature, that is, the mold surface temperature rxT in equation (1) above, as seen in FIG. 1, the Ni content Jul is 2%! In the region below 1 degree, when the mold surface temperature is low, the ductility of the mold surface layer tends to be low, and delamination mainly consisting of brittle cracks progresses. Therefore, for example, 21? When the surface temperature of the mold during forging decreases due to a decrease in the heavy electrical current of the product and a shortening of the contact time with the mold, it is necessary to ensure the ductility of the surface layer of the mold.

On the other hand, when the mold surface temperature is high, ductility can be ensured as stated in the specification of patent El (J4 (1) for rA forging fate river) filed in April 1986 by the present inventor. It is necessary to determine the upper limit of the Ni content from the viewpoint of avoiding a decrease in toughness rather than reducing the toughness, and the above formula (1) cannot be applied.

From the above, the mold according to the present invention is applied when the mold surface temperature is relatively low, and the temperature range is 700°C, which corresponds to a relatively low mold surface temperature, and 700°C, which corresponds to a relatively high mold surface temperature. The boundary temperature was defined as a temperature near the middle between 780°C and 750°C.

Example The following will explain based on examples.

After melting alloy steel having the composition of the present invention and subjecting it to heat treatment of quenching and tempering, a flat mold with a diameter of 500 m and a height of 700 m was made, and heated to 1250 ° C. 1,000 pieces of carbon steel were forged, and the presence or absence of cracks in the mold was investigated, and the amount of wear on the surface layer of the mold after forging was measured. The mold surface temperature is 700°C. In addition, to measure the amount of wear, a sample material for measuring the amount of wear made of the same material as the mold used for the test and having a diameter of 50 mm and a height of 50 mm was embedded in the surface of the mold. After completing the 81-year construction, it was taken out and the amount of wear on the surface was determined. For comparison, a composition that deviates from the composition of the present invention ("
Similar tests were conducted on comparative steel. Table 1 shows the chemical composition of the sample materials.

As a result of the test, no cracking in the mold was observed in either the inventive steel or the comparative steel. As shown in Table 1 above, the amount of wear in the steel of the present invention is about half that of the comparative steel, indicating that the wear resistance is significantly improved. In addition, conventional steel is compared @ (22), (23)
It has a composition close to that of

As described in detail of the invention, instead of the alloy steel that was conventionally used as a material for hot-operation molds and which was made to have high strength by adding precipitation-strengthening alloy elements such as Mo, NN was added. At the same time, the content of other ingredients is also set within an appropriate range. By using 8I steel for mold making, it is possible to prevent the mold from cracking without reducing toughness due to high strength, and the wear resistance is significantly improved, significantly extending the life of the mold. can.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between Ni-containing affi and elongation of a hot@operation mold material. Figure 1 Ni officials, etc. t<x)

Claims (1)

[Claims] In weight%, C: 0.50 to 0.70%, Si: 0.50
% or less, Mn: 0.20-0.90, P: 0.035%
Below, S: 0.015% or less, Cr: 0.50 to 2.0
%, Mo: 0.20-0.60%, V: 0.04-0.
30%, acid-soluble Al: 0.10% or less, and further contains Ni, where T is the surface temperature of the mold during hot forging (°C), -0.025T+19≦Ni≦-0. 1. Steel for hot forging dies, characterized in that it contains alloy steel that satisfies 025T+21, and the remainder is Fe and unavoidable impurities.